eh_frame confusion

Naveen N. Rao wrote:
> Rasmus Villemoes wrote:
>> I'm building a ppc32 kernel, and noticed that after upgrading from gcc-7
>> to gcc-8 all object files now end up having .eh_frame section. For
>> vmlinux, that's not a problem, because they all get discarded in
>> arch/powerpc/kernel/vmlinux.lds.S . However, they stick around in
>> modules, which doesn't seem to be useful - given that everything worked
>> just fine with gcc-7, and I don't see anything in the module loader that
>> handles .eh_frame.
>> 
>> The reason I care is that my target has a rather tight rootfs budget,
>> and the .eh_frame section seem to occupy 10-30% of the file size
>> (obviously very depending on the particular module).
>> 
>> Comparing the .foo.o.cmd files, I don't see change in options that might
>> explain this (there's a bunch of new -Wno-*, and the -mspe=no spelling
>> is apparently no longer supported in gcc-8). Both before and after, there's
>> 
>> -fno-dwarf2-cfi-asm
>> 
>> about which gcc's documentation says
>> 
>> '-fno-dwarf2-cfi-asm'
>>      Emit DWARF unwind info as compiler generated '.eh_frame' section
>>      instead of using GAS '.cfi_*' directives.
>> 
>> Looking into where that comes from got me even more confused, because
>> both arm and unicore32 say
>> 
>> # Never generate .eh_frame
>> KBUILD_CFLAGS           += $(call cc-option,-fno-dwarf2-cfi-asm)
>> 
>> while the ppc32 case at hand says
>> 
>> # FIXME: the module load should be taught about the additional relocs
>> # generated by this.
>> # revert to pre-gcc-4.4 behaviour of .eh_frame
> 
> Michael opened a task to look into this recently and I had spent some 
> time last week on this. The original commit/discussion adding 
> -fno-dwarf2-cfi-asm refers to R_PPC64_REL32 relocations not being 
> handled by our module loader:
> http://lkml.kernel.org/r/20090224065112.GA6690@bombadil.infradead.org
> 
> However, that is now handled thanks to commit 9f751b82b491d:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9f751b82b491d
> 
> I did a test build and a simple module loaded fine, so I think 
> -fno-dwarf2-cfi-asm is not required anymore, unless Michael has seen 
> some breakages with it. Michael?
> 
>> 
>> but prior to gcc-8, .eh_frame didn't seem to get generated anyway.
>> 
>> Can .eh_frame sections be discarded for modules (on ppc32 at least), or
>> is there some magic that makes them necessary when building with gcc-8?
> 
> As Segher points out, it looks like we need to add 
> -fno-asynchronous-unwind-tables. Most other architectures seem to use 
> that too.

Can you check if the below patch works? I am yet to test this in more 
detail, but would be good to know the implications for ppc32.

- Naveen


---

On 02/03/2020 18.32, Naveen N. Rao wrote:
> Naveen N. Rao wrote:
>> Michael opened a task to look into this recently and I had spent some
>> time last week on this. The original commit/discussion adding
>> -fno-dwarf2-cfi-asm refers to R_PPC64_REL32 relocations not being
>> handled by our module loader:
>> http://lkml.kernel.org/r/20090224065112.GA6690@bombadil.infradead.org
>>
>> However, that is now handled thanks to commit 9f751b82b491d:
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9f751b82b491d
>>
>>
>> I did a test build and a simple module loaded fine, so I think
>> -fno-dwarf2-cfi-asm is not required anymore, unless Michael has seen
>> some breakages with it. Michael?
>>
>>>
>>> but prior to gcc-8, .eh_frame didn't seem to get generated anyway.
>>>
>>> Can .eh_frame sections be discarded for modules (on ppc32 at least), or
>>> is there some magic that makes them necessary when building with gcc-8?
>>
>> As Segher points out, it looks like we need to add
>> -fno-asynchronous-unwind-tables. Most other architectures seem to use
>> that too.

Yes. Thanks, Segher, that explains that part.

> Can you check if the below patch works? I am yet to test this in more
> detail, but would be good to know the implications for ppc32.

I'll see if that produces a bootable kernel, but I think I'd prefer a
more piecemeal approach.

One patch to add -fno-asynchronous-unwind-tables (given that other
arches do it unconditionally I don't think cc-option is needed), with a
commit log saying something like "no-op for gcc < 8, prevents .eh_frame
sections that are discarded anyway for vmlinux and waste disk space for
modules". Then another patch can get rid of -fno-dwarf2-cfi-asm if
that's no longer required.

Rasmus

Naveen N. Rao wrote:
> Naveen N. Rao wrote:
>> Rasmus Villemoes wrote:
<snip>
> Can you check if the below patch works? I am yet to test this in more 
> detail, but would be good to know the implications for ppc32.
> 
> - Naveen
> 
> 
> ---
> diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
> index f35730548e42..5b5bf98b8217 100644
> --- a/arch/powerpc/Makefile
> +++ b/arch/powerpc/Makefile
> @@ -239,10 +239,7 @@ KBUILD_CFLAGS += $(call cc-option,-mno-vsx)
>  KBUILD_CFLAGS += $(call cc-option,-mno-spe)
>  KBUILD_CFLAGS += $(call cc-option,-mspe=no)
>  
> -# FIXME: the module load should be taught about the additional relocs
> -# generated by this.
> -# revert to pre-gcc-4.4 behaviour of .eh_frame
> -KBUILD_CFLAGS  += $(call cc-option,-fno-dwarf2-cfi-asm)
> +KBUILD_CFLAGS  += $(call cc-option,-fno-asynchronous-unwind-tables)

In terms of the CFI information, the primary difference I see with 
-fno-dwarf2-cfi-asm is that when dumping the debug frames, CIE indicates 
version 3, while otherwise (i.e., without -fno-dwarf2-cfi-asm and 
with/without -fasynchronous-unwind-tables), it is version 1, regardless 
of -gdwarf-2/-gdwarf-4. There are few more minor changes, but none of 
these looked significant to me.

>  
>  # Never use string load/store instructions as they are
>  # often slow when they are implemented at all
> diff --git a/arch/powerpc/kernel/vdso32/Makefile b/arch/powerpc/kernel/vdso32/Makefile
> index e147bbdc12cd..d43b0b18137c 100644
> --- a/arch/powerpc/kernel/vdso32/Makefile
> +++ b/arch/powerpc/kernel/vdso32/Makefile
> @@ -25,6 +25,7 @@ KCOV_INSTRUMENT := n
>  UBSAN_SANITIZE := n
>  
>  ccflags-y := -shared -fno-common -fno-builtin -nostdlib \
> +       -fasynchronous-unwind-tables \
>         -Wl,-soname=linux-vdso32.so.1 -Wl,--hash-style=both
>  asflags-y := -D__VDSO32__ -s
>  
> diff --git a/arch/powerpc/kernel/vdso64/Makefile b/arch/powerpc/kernel/vdso64/Makefile
> index 32ebb3522ea1..b2cbb5c49bad 100644
> --- a/arch/powerpc/kernel/vdso64/Makefile
> +++ b/arch/powerpc/kernel/vdso64/Makefile
> @@ -13,6 +13,7 @@ KCOV_INSTRUMENT := n
>  UBSAN_SANITIZE := n
>  
>  ccflags-y := -shared -fno-common -fno-builtin -nostdlib \
> +       -fasynchronous-unwind-tables \
>         -Wl,-soname=linux-vdso64.so.1 -Wl,--hash-style=both
>  asflags-y := -D__VDSO64__ -s

The above vdso hunks can be dropped since all our VDSO are assembly, so 
the above have no impact.


- Naveen